1. Field of the Invention
The present invention relates to a process for producing an exhaust-gas-purifying catalyst which is capable of simultaneously purify three components, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x), included in exhaust gases emitted from internal combustion engines for automobiles etc. An exhaust-gas-purifying catalyst produced in accordance with the present invention is especially useful for applications in fuel-lean atmosphere in which oxygen is included in an amount of the stoichiometric point, or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) in exhaust gases.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x simultaneously to purify exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a porous catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the porous catalyst carrier layer.
Recently, it has been likely to install an exhaust-gas-purifying catalyst directly below a manifold which is disposed adjacent to an engine, and accordingly an exhaust-gas temperature has been likely to increase during high-speed traveling. As a result, an exhaust-gas-purifying catalyst has been more likely to be exposed to elevated temperatures. However, in conventional exhaust-gas-purifying catalysts, thermal degradation might be developed in the gamma-alumina by high-temperature exhaust gases. The sintering of the noble catalyst ingredient resulting from the thermally-degraded gamma-alumina might possibly deteriorate the catalytic capability of conventional exhaust-gas-purifying catalysts.
In order to solve the possible problems associated with conventional exhaust-gas-purifying catalyst, Japanese Unexamined Patent Publication (KOKAI) No. 4-122,441 discloses a novel process for producing an exhaust-gas-purifying catalyst. In the novel production process, alumina is utilized which has been thermally treated in advance, and a noble metal catalyst ingredient is loaded on the thermally-treated alumina. Since the alumina has been thermally treated in advance by the novel production process, the thermal degradation is hardly developed in the resulting exhaust-gas-purifying catalyst even when it is exposed to elevated temperatures, and accordingly the noble metal catalyst ingredient is little sintered. Thus, the resulting exhaust-gas-purifying catalyst can stably exhibit its catalytic capability.
When driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines.
However, it has been found that the exhaust-gas-purifying catalyst produced by the novel process disclosed in the aforementioned patent publication considerably suffers from the sintering of noble metal catalyst ingredient when it is operated under the fuel-lean conditions and when it is subjected to elevated temperatures of 800.degree. C. or more. Thus, the exhaust-gas-purifying catalyst is degraded in terms of catalytic capability.